The present invention relates to an apparatus for controlling ink supply amounts of individual zones obtained by dividing an axial length of a printing plate mounted on a plate cylinder in a printing press.
In a conventional printing press, blades corresponding to the individual zones of the printing plate are disposed opposite to an ink fountain and an ink fountain roller which are used to continuously supply optimal amounts of ink to the printing plate mounted on the plate cylinder. Gaps between the ink fountain roller and the respective blades are independently adjusted in accordance with an image pattern on the printing plate, and the amounts of ink supplied to the individual zones are determined. This operation is performed by remote control. For this purpose, gap adjusting motors are arranged for the blades, respectively, and potentiometers are also arranged to convert gaps into electrical signals. Gap signals are supplied to feedback signals to a DC servo control circuit, thereby controlling the ink supply amounts of the individual zones.
With the above arrangement, control precision and stability are insufficient due to durability of servo motors and potentiometers, their deterioration over time, variations in impedances of connecting wires, and electrical noise.
In order to eliminate the above drawbacks, stepping motors and a digital control circuit can be used to perform open or closed loop control in place of the above analog control.
In open loop control, however, when each stepping motor cannot be rotated in synchronism with drive pulses and becomes out of step, amounts of ink cannot be appropriately supplied. In the worst case, such control becomes inaccurate or impossible, resulting in defective printing. Therefore, it is necessary to use closed loop control.
A control origin is detected by a sensor, and control is performed in accordance with the detected control origin. Alternatively, a gap between the blade and the ink fountain roller is detected as an electrical signal in the same manner as in the conventional technique, or a rotational condition of each motor is detected by a rotary encoder or the like, and a detection output is used as a feedback signal.
In closed loop control, when the control origin is detected by the sensor, control values excluding the original control value are not given by closed loop control. The out-of-step state of the stepping motor cannot be detected. When the potentiometer is used, its durability is degraded because it has mechanical contacts. At the same time, detection precision is limited, and high-stability, high-precision control is impossible. When the rotary encoder or the like is used, the out-of-step state of the stepping motor causes rotational torque pulsation and hunting. A hunting pulse is generated by the rotary encoder or the like. A count of the rotary encoder becomes inaccurate, and a control error occurs.